The ultrasound imaging technology is a technology that takes images of the inside of a test object such as a human body noninvasively using ultrasound waves (inaudible sound waves, that is, sound waves whose frequencies are typically 20 kHz or higher).
As a transmission method for transmitting an ultrasound beam from an ultrasound probe to a test object such as a human body, there are two kinds of transmission methods, and one is a dispersing-type transmission method in which an ultrasound beam that disperses in a fan shape is transmitted, and another is a focusing-type transmission method in which the transmit focus of an ultrasound beam is disposed inside of a test object, and the ultrasound beam is converged on the focus.
Because the transmission/reception of ultrasound waves by an ultrasound image pickup apparatus is performed by means of an array with an aperture of a finite diameter, the transmission or reception is affected by the diffractions of the ultrasound waves caused by the edge of the aperture, therefore it is difficult to improve the resolution in the direction of an azimuthal angle. The above problem can be solved if an array of an infinite length can be prepared, but in actuality it is impossible to prepare an array of an infinite length. Therefore, in order to improve the resolution in the direction of an azimuthal angle, channel domain phasing technologies have been widely studied in recent years, with the result that new phasing schemes such as an adaptive beamformer and aperture synthesis have been extensively reported.
The aperture synthesis will be briefly explained. First, by respectively giving delay times to reception signals received by plural elements included in an ultrasound probe, the delayed reception signals are virtually focused on a certain point, and then a phased signal is obtained by adding these delayed reception signals. The aperture synthesis is performed by synthesizing this phased signal and one phased signal or more obtained regarding the same point through other one or more transmissions/receptions, and by superimposing these signals on each other.
In the aperture synthesis, because phased signals obtained by an ultrasound probe through the transmission/reception to or from different directions regarding a certain point can be superimposed on each other, it can be expected that the improvement of the resolution of a point image and the robustness against the inhomogeneity of the point image are provided. In addition, because processing gain can be increased owing to the superimposing processing, the number of transmissions of ultrasound waves can be reduced in comparison with the number of usual transmissions of ultrasound waves, the aperture synthesis can be also applied to high-speed imaging.
Patent Literature 1 relates to an ultrasound diagnostic apparatus, and discloses a technology in which aperture synthesis is performed using an improved virtual source method in ultrasound imaging in which focusing-type transmission is performed. To put it concretely, the aperture synthesis is performed under the assumption that a focus is a virtual source in an area where the energy of an ultrasound beam is converged on a focus (an area A shown in FIG. 2 of Patent Literature 1), while the aperture synthesis is performed under the assumption that a spherical wave is irradiated from the end of a probe in areas which are adjacent to the area A and in which the energy of the ultrasound beam disperses (areas B and C).